In a number of industries, the methods for producing finished metal products by processes such as machining, transformation and cutting of intermediate products generate large amounts of machine scraps or chips. In this text, we refer to this waste with the term “scrap”. The aeronautics and space industries in particular generate a large amount of scrap, because numerous aircraft components and structural parts are obtained by integral machining of massive parts. However, the scrap is often difficult to recycle directly due to the mixture with other alloys of aluminum or other materials used in aeronautics, such as stainless steel and titanium. The waste from machining of aeronautics alloys is thus, for example, recycled into aluminum-silicon alloys intended for molded products. This method of upgrading will hereinafter be referred to as the “usual stream”. It results in a partial loss in the value of the metal.
It is also possible to produce products made of aluminum alloys, from scrap, intended for aeronautical and space applications. Patent FR 2801060 thus describes the production of products and semi-finished products in 7000 series alloys using raw materials for recycling, such as machine scraps or chips. This recycling is made possible by a step of purifying certain impurities. This method of upgrading will hereinafter be referred to as the “aeronautic stream”.
The term “aluminum-lithium-type alloy” refers to any alloy containing an amount of lithium greater than 0.2% by weight and preferably greater than 0.5% by weight, regardless of the content of the other alloying elements. The common feature of this alloy family is the need for specific precautions associated with the oxidizability of lithium. Aluminum-lithium (Al—Li) alloys have a lower density than conventional aluminum alloys and are therefore highly beneficial for reducing the weight of aeronautical parts. However, Al—Li type alloys are significantly more expensive than products made of alloys not containing lithium, for three reasons: the intrinsic cost of lithium, the cost of development of the metal in foundry, and the difficulty of recycling the scrap. This invention relates to the recycling of the scrap of Al—Li type alloys, and in particular finely divided scrap. This scrap is generated not only during the production of structural elements from semi-finished products that are rolled, drawn or forged, but already in the aluminum plant itself, during the scalping of ingots for rolling or during the skinning of billets.
According to the prior art, only massive Al—Li type alloy waste is recycled, such as ingots for rolling, billets, rejected heavy plates or waste from cropping of heavy plates. With regard to finely divided scrap such as machine chips of an Al—Li type alloy, those skilled in the art tend to believe that the recycling of this scrap is not possible in industrial amounts. Thus, in the “usual stream”, the presence of lithium may result in numerous problems. First, from a practical point of view when melting the scrap, lithium promotes oxidation of the liquid metal and consequences include the presence of fumes, the formation of thick crusts and possibly complete solidification of the melt. Second, lithium is a detrimental metal for many aluminum alloy applications, such as packaging (in which, for certain applications, the residual content of lithium should not exceed 0.0001%), or because the maximum allowable content of any impurity is 0.05% (Robare and Witters, Light Metals 1991, p. 1223). Similarly, in the “aeronautical stream”, it is generally accepted (Wilson et al. Journal de Physique C-75 1987) that the recycling of Al—Li type alloy scrap in the same alloys is not possible under the quality conditions required by the industries that use these products, in particular the aeronautics and space industries.
In the prior art, the most common solution enabling the scrap of Al—Li type alloys to be recycled consists of using a method enabling aluminum to be separated from lithium (Wilson et al., Rabare and Witters). Thus, there are numerous patent applications relating to the purification of liquid mixtures of aluminum-lithium by electrolytic methods. Methods of this type are described in U.S. Pat. No. 4,780,186 (Alcoa), U.S. Pat. No. 4,790,917 (Alcan), U.S. Pat. No. 4,849,072 (Alcoa), U.S. Pat. No. 4,882,017 (Alcoa), U.S. Pat. No. 4,973,390 (Alcoa) and U.S. Pat. No. 5,071,523 (Alcoa). Other chemical-type solutions have also been proposed. FR 2600343 (Pechiney) proposes a method for recovering aluminum and lithium from metal waste by chlorination of lithium and recovery of the lithium chloride formed. Similarly, U.S. Pat. No. 4,758,316 (Alcoa) proposes a method for remelting and purifying Al—Li type alloy scrap, in particular by forming a LiCl salt by chlorination of the liquid metal. U.S. Pat. No. 5,032,171 (Alcoa) proposes a method for removing lithium in a molten aluminum-lithium alloy by the addition of a hydrofluoric acid salt and electromagnetic stirring.
According to the prior art, a large amount of the finely divided scrap of the Al—Li type alloys therefore leaves the aluminum industrial circuit for final use in external systems, for example, to deoxidize steel. This weighs heavily on the overall economy of Al—Li alloys and inhibits their economic development. This economic effect is further reinforced when Al—Li type alloys contain other expensive chemical elements, such as silver, scandium, rare earth metals, and copper. By way of example, the alloy AA 2195 contains between 0.8 and 1.2% lithium and between 0.25 and 0.6% silver, and the alloy AA 2098 contains between 0.8 and 1.3% Li and between 0.25 and 0.6% silver. In general, it is desirable to improve the rate of recycling of Al—Li type alloys.
The prior art describes methods for melting aluminum-alloy scrap in general. U.S. Pat. No. 3,999,980 describes a method for melting scrap in an inert atmosphere and compares it with typical methods using a molten salt. U.S. Pat. No. 4,159,907 shows the problems related to the low scrap density, which results in a tendency for the scrap to remain at the surface of liquid aluminum and to be oxidized, and proposes densifying the scrap by compression in order to overcome this problem. U.S. Pat. No. 6,074,455 describes a method for rapid immersion of scrap into the liquid aluminum by adding it to the vortex created by a rotor. U.S. Pat. No. 3,873,305 describes a melting process for container scrap in which scrap is rapidly melted by forced submergence in a moving stream. Japanese Patent No. 10147822 describes a furnace used for scrap remelting in which scrap is stirred above the melt with a specific apparatus. U.S. Patent Application 2004/0244535 proposes adding waste of Al—Li type alloys in densified form to a tundish suitable for achieving rapid immersion.
To melt scrap, it is therefore preferable, according to the prior art, to perform a rapid immersion or protection by an inert atmosphere or with molten salt so as to avoid oxidation of the scrap. For Al—Li type alloys, which are even more sensitive to oxidation than other aluminum alloys, a person skilled in the art will consider it necessary to push the methods of rapid immersion or protection by an inert atmosphere or with molten salt even further, and all the more so if finely divided scrap, i.e. having a high surface area, is used. It is thus mentioned in the prior art (D. Naser, Material Eng. 103(4) p. 48) that the problem of mixing Al—Li type alloy scrap during melting is unsolved and that special equipment is needed for melting this type of scrap.
A problem that this invention is intended to solve is therefore that of providing a novel approach to the production of aluminum-lithium-type alloy remelting ingots to enable a significant fraction of finely divided scrap to be incorporated.